From ReligiousTolerance.org:
The term Unitarian has traditionally had two main religious meanings:
A monotheistic belief that God is a unity, not a trinity. This was widespread in the early Christian movement, The exact nature and makeup of deity occupied the thoughts of many Christians during the first few centuries CE. There were many anti-trinitarian movements at the time. Three were monarchianism, sabellianism and patripassianism. A series of church councils decided that God is a Trinity, composed of the Father, Son, and Holy Spirit. Unitarianism then became a heresy and was suppressed. This belief system still exists in the form of Biblical Unitarianism: Christians who believe "in one God, the Father, and one Lord, Jesus Christ...The "Holy Spirit" is another name for God..." 3
A contemporary religious movement which features a lack of dogma, a belief in the inherent goodness of people, and the obligation for each member to seek out and develop his or her own system of beliefs and ethics.
It is this second meaning of Unitarian that we will use here.
The term Universalism has also had two religious meanings:
The belief that Jehovah as described in the Hebrew Scriptures (Old Testament) is the deity for all humanity, rather than just for the Jewish people.
A religious movement which historically promoted the belief that every person will go to heaven after death. This is in contrast with the traditional Christian belief that one's natural destination is eternal torment in hell. Only those who are saved will attain heaven. Today, the latter beliefs are still held by some conservative Christians. Other mainline and conservative Christians are drifting toward the Universalist belief. Liberal and most mainline Christians are already there.
Only a few consider themselves to be atheist:
According to a 1997 survey of almost 10,000 UUs gave their theological perspective as:
46.1% Humanist. This is the most common belief system.
19% identify themselves as Nature or Earth centered religion (e.g. Wiccan, Druid or other Neopagan tradition.
13% describe themselves simply as Theist.
9.3% self-identify as Christian.
6.2% are mystic.
3.6% are Buddhist.
Other perspectives listed are Jewish at 1.3%, Hindu at 0.4%, Muslim at 0.1% and other at 13.3%
And most consider themselves to be brought together by their views on shared values and respect for one another as individuals.
I wouldn't think of it so much as a religion, but more a view of life based on what I read.